When pure pigments are mixed into base paints, dust formation results in loss of pigment material and affects health and safety circumstances. Moreover, after mixing pure pigment into a base paint, the pigment particles tend to stick together and form lumps. The pigments do not disperse properly and do not develop their full colour strength. To overcome this problem colour mixing systems generally make use of pigment pastes, as is the case in the system disclosed in EP-A 0 311 209. The composition of such a pigment paste generally depends on the type of base paint: Pigment pastes for universal application (aqueous as well as solvent borne paints) typically comprise co-solvents, e.g. glycols, water, surfactants, dispersants, and optionally further additives. Pigment pastes solely for use in solvent borne paints typically comprise a substantial content of a polymeric binder, solvents, and additives. In the pastes, the pigments are pre-dispersed and further ground and milled. Such pastes are highly coloured and are filled into containers for distribution.
Colour mixing at points of sale, for instance in a consumer shop, involves the use of a colour mixing apparatus. Such a mixing apparatus typically comprises a carousel of canisters filled with pigment pastes. The pastes are transferred to the canisters from the containers in which they were distributed, ready for use as tinters. Each canister can be positioned above a container with a base paint. The canisters have an opening at their lower side, shut by a shut-off valve. A central processing unit can open the valve and dose the pigment paste gravimetrically or volumetrically into the base paint, in accordance with a formula corresponding to a selected paint colour. Examples of such devices are disclosed in EP-A 1 109 678. The machines are calibrated for each paste. If the viscosity of the paste changes over time, then the calibration of the machine becomes less accurate, leading to inaccurate tinting.
After use, the distribution containers remain contaminated with a small amount of strongly coloured material, making them unsuitable for disposal as conventional waste.
Due to their composition, the pigment pastes are not universally compatible with all types of base paints. If a pigment paste comprises a binder, it may be incompatible with the binder of a base paint. Further, the additives in the pastes, particularly surfactants and dispersants, can have a negative effect on the film properties of the paint after application. The solvent or co-solvent content of the pigment pastes increases the volatile organic content of the mixed paint. Moreover, since the pigment pastes also comprise non-colorant materials, the colour strength of the pigments is diluted.
It has been attempted to overcome these problems by using solid pigment concentrates, e.g., in the form of tablets such as in EP-A 0 121 308, or granulates, such as in WO 03/064540. In practice, these concentrates are found to be more suitable for water borne base paints than for solvent borne base paints and the colour strength remains a critical factor.
U.S. Pat. No. 6,111,054 discloses solid pigment concentrates with polyamine dispersants obtainable by the reaction of organic acids with polyamines. The disclosed dispersants are amphiphilic dispersants. The organic acids in one pendent side group comprise water-compatible blocks, such as polyethers, and hydrophobic solvent-compatible blocks, such as polycaprolactone. Drawbacks are the inferior dispersibility and the lower colour strength obtained from pigment compositions prepared from such dispersants.